This invention relates to the post-weld heat treatment of thin wall metal structures. In particular the invention relates to a method of locally heat treating a weld seam without thermally affecting material adjacent to the weld.
The shells of typical positive expulsion propellant fuel tanks for spacecraft that incorporate rolling diaphragms are fabricated by welding metal domes or domes and cylinders together. Additionally, the diaphragm and its supporting structure is typically welded into the tank shell components prior to final shell assembly. Of the many important design criteria associated with a spacecraft, an overriding design driver is the need for low mass. The need for reduced mass drives the material choices and the wall thickness of the tank design. In the area of the welds that hold the domes and/or cylinders together, the wall thickness is often greater than the rest of the tank wall because of the reduced strength and toughness available in the welds and the adjacent heat affected zone. Much of the available tensile strength can be restored through the use of post-weld heat treatment (PWHT) for recovery of ductility and/or stress relief.
While the PWHT of the tank shell can be readily accomplished using industry practices such as vacuum heat treatment and retort heat treatment with the part sealed in a container of inert gas, many tanks utilize metallic rolling diaphragms to separate the propellants from pressurants to enable positive expulsion of propellants in microgravity. The fragile diaphragm materials may be damaged by exposure to the high temperatures required for thermal stress relief during PWHT, which prevents the use of conventional furnaces to raise the temperature of the entire tank to the stress relief temperatures. Without stress relief the reduced material properties and residual tensile stresses remain in the weld areas of the tank shell. As a result, the tank must be made thicker to provide sufficient safety margin. The increased thickness and resultant increased mass is detrimental to the utility of the tank.